This invention relates to a heterojunction bipolar transistor having an emitter region of a semiconductor material with a band gap greater than that of a semiconductor material of which a base region is formed.
The recent advance of an MBE (molecular beam epitaxy) and an MOCVD (metal organic chemical vapor deposition) method leads to the active development of semiconductor devices utilizing a heterojunction, i.e., a junction between dissimilar semiconductor materials. The heterojunction bipolar transistor with a heterojunction between the base and the emitter has many advantages over a conventional bipolar transistor with a homojunction formed of a single semiconductor material.
The advantages of the heterojunction bipolar transistor has, for example, the following advantages:
(1) Since an emitter region is formed of a semiconductor material having a band gap greater than that of a semiconductor material of which a base region is formed, it is possible to obtain a high emitter injection efficiency even if an impurity concentration ratio of the emitter region to the base region is smaller.
(2) As a result of (1), the impurity concentration level of the base region can be increased, thus enabling a decrease in a base resistance.
(3) As a result of (1), the impurity concentration level of the emitter region can be lowered, thus enabling a decrease in an emitter capacity.
As evident from these advantages, the heterojunction bipolar transistor is potentially, excellent in high frequency and switching characteristics over the homojunction bipolar transistor.
Of the semiconductor materials capable of actually providing such heterojunction bipolar transistor, a semiconductor material of a broader band gap is ordinarily smaller in electron affinity. As a result, if an npn transistor is manufactured which has stepped junctions formed directly between the emitter of a material offering a broader band gap and the base of a material offering a narrower band gap, then the injection efficiency will be lowered for the reason set out below. Thus, no advantage can be made of the merits of the heterojunction bipolar transistor.
FIG. 1 shows the state of a conduction band in the neighborhood of the base-to-emitter junction of a stepped heterojunction bipolar transistor in which the electron affinity of a semiconductor material of which the emitter region is formed is smaller by .DELTA.E.sub.C than that of a semiconductor material of which the base region is formed. As evident from FIG. 1, since a spike-like barrier of a level .DELTA.E.sub.C is formed in the conduction band due to a difference between the electron affinities mentioned, the electron injection from the emitter into the base region is blocked due to the presence of such barrier, causing a decline in emitter injection efficiency and a consequent degradation in the characteristic of the heterojunction bipolar transistor.
This problem is overcome by forming a graded heterojunction, in place of the stepped heterojunction, between the emitter and the base with a band gap smoothly varying from the emitter toward the base. FIG. 2 shows the state of the conduction band when, between the base and the emitter, a region is formed with a band gap varying in a linear fashion. As appreciated from FIG. 2, barriers produced in the conduction band can be removed using the graded heterojunction.
A region (hereinafter referred to as a transition region) with a band gap varying between the emitter region and the base region has a composition intermediate between the composition of a semiconductor material of which the base region is formed and that of a semiconductor material of which the emitter region is formed. In order for the band gap to be varied linearly, or smoothly in a quadratic fashion, in the transition region the composition ratio of the semiconductor material is gradually varied as shown in FIG. 3. In order to create such a transition region it is necessary to continuously and exactly vary the flux of a molecular beam for the MBE method or an amount of gas stream for the MOCVD method. In the present techniques, however, such control is difficult to attain and a high cost is required in providing such a control device. Furthermore, a transition region should be grown at an adequately slow speed, requiring a high manufacturing cost. When, moreover, the transition region is formed with a band gap varying smoothly, holes are liable to be injected from the base region into the transition region. In consequence, carrier recombinations are increased when, in particular, a shorter carrier lifetime is involved. This causes a drop in the DC current gain and an attendant degradation in high frequency and switching characteristics due to an increase in stored carriers. Therefore, a problem arises from the fact that the advantages of heterojunctions are impaired in the conventional heterojunction bipolar transistor.